Glioblastoma multiforme (GBM), the most common primary brain cancer, has a 5-year survival rate of <12%. GBM treatment is severely limited by the fact that chemotherapeutic drugs reach the brain in very low concentrations due to the blood brain barrier (BBB). Current strategies to circumvent the BBB (i.e. Gliadel wafers and convection enhanced delivery) are invasive and lead to only moderate improvements in survival. Clearly, less-invasive strategies that provide sustained and well-dispersed drug delivery to brain tumors are needed. To address this need, we propose an innovative image guided drug-delivery approach that couples magnetic resonance (MR)-targeted BBB opening via focused ultrasound (FUS) and microbubbles (MBs) with drug-loaded nanoparticles that have been engineered with extremely dense polyethylene glycol (PEG) coatings to rapidly penetrate brain tissue (i.e. brain penetrating nanoparticles or BPNs). We hypothesize that this approach will improve brain cancer treatment by enhancing drug delivery across the BBB to FUS-targeted tumors, providing sustained drug delivery deep within tumors, and minimizing systemic side effects. We will test this hypothesis with 4 aims. In Aim 1, we will optimize BPN size and surface chemistry for brain tumor penetration and long circulation time. Tracer BPNs will be used to determine the nanoparticle size range and surface chemistry required to produce BPNs with controlled particle penetration depths in freshly obtained brain tumors ex vivo. Subsequently, BPNs with these optimal characteristics will be generated from biodegradable polymers and tested. In Aim 2, running in parallel with Aim 1, we will determine FUS pressure thresholds for safe and reversible MR-guided BBB opening to gadolinium in intracranial 9L rat brain tumors as a function of MB diameter. Then, FUS pressure thresholds will be used as a basis for determining optimal FUS and MB parameters for delivering the most promising BPN formulation from Aim 1 to brain tumors. These FUS and MB parameters will be carried to Aim 3, wherein we will evaluate whole-body and brain biodistributions of biodegradable BPNs via Fluorescence Molecular Tomography (FMT). BPN dispersion into the brain after delivery across the BBB will be further analyzed in detail using confocal microscopy. Brain tissue inflammation, which is expected to be negligible or absent, will be assessed histologically. In Aim 4, we will first determine the maximum tolerated dose (MTD) of paclitaxel-loaded BPNs, followed by an evaluation of drug pharmacokinetics (PK). Finally, we will determine the overall efficacy of drug-loaded BPN delivery with MR- guided FUS and MBs to invasive brain tumors by measuring reduced tumor growth and enhanced animal survival after treatment. If these pre-clinical studies proceed as expected, we are well-positioned for translation to the clinic. Our next step would be to test the safety of the MR-guided BPN delivery approach in large animals (pig) using the University of Virginia's clinical Insightec Exablate system, followed by the initiation of a clinical trial.